Aperiodic high-frequency voltmeter



1950 H. J. LINDENHOVIUS 2,525,039

APERIODIC HIGH-FREQUENCY VOLTMETER Filed July 20, 1946 Patented Oct. 10, 1950 APERIODIC HIGH-FREQUENCY VOLTMETER Hendrik Jan Lindenhovius, Eindhoven, Netherlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application July 2-0, 1946, Serial No. 685,171

In the Netherlands November 11, 1943 Section 1, Public Law'690, August 8, 1946 Patent expires November 11, 1963 4 Claims. (01. 171' 95) This invention relates to apparatus adapted to measure the amplitude of a high frequency signal of any given frequency.

For measuring the amplitude of a high-irequency signal of any frequency aperiodic high-.

frequency voltmeters are known in which the high-frequency voltage to be measured and an auxiliary voltage of lower frequency are'supplied to a mixing tube and by detection of the modulated voltage thus obtained the auxiliaryvoltage is again obtained, the amplitude of which, however, by now increases with that of the highfrequency signal to be measured, the latter aux iliary voltage controlling an indicating instrument after being selectively amplified.

According to the .invention, an improvement of an aperiodic high-frequency voltmeter of the said kind is obtained by using a potentio'meter instead of a mixing tube as the modulator, the said potentiometer having supplied across its input terminals the high-frequency voltage to be measured, the potentiometer ratio being varied at a rate corresponding to the frequency of the auxiliary Voltage, which is preferably of low frequency. In this manner, the high frequency voltage appearing at the output terminals of the potentiometer is amplitude modulated at the frequency of the auxiliary voltage. It should be noted that in the absence of a high frequency voltage at the input terminals, no voltage appears at the output terminals .of the potentiometer modulator whereas with the conventional mixing tube circuit, in the event where only a low frequency voltage is impressed on the inputthereof,

v a low frequency voltage will be yielded in the output.

In the said well-known voltmeter, the output circuit of the mixing valvev should include a filter suppressing the auxiliary Voltage frequencies in order to avoid the occurrence across the detector of a low-frequency voltage in the absence of a high-frequency voltage to be measured.

In the use of the invention there is no need for such a filter since in the absence of a highfrequency signal the detector has not supplied to it any signal. of auxiliary-voltage frequency. Thus, not only are avoided the disadvantages generally accruing to a mixin circuit, such as variation with supply voltages and eventual variation of the tube properties, but the advantage is also obtained that by the absence of a filter between modulator and detector thedamping varying with frequency which is otherwise due thereto is avoided and hencethe sensitiveness of the aperiodic voltmeter at comparatively low frequencies is not unfavourably influenced.

The high-frequency signal to be measured is preferably supplied to the potentiometer, which acts as a modulator, via an aperiodic amplifying and/ or blocking stage comprisin an amplifier tube. As by now the amplifier tube does not serve for modulation it can be adjusted'to maximum amplification by a proper choice of the circuit elements or the like. It must be mentioned thatthe amplification factor thus realized may be lower than 1 if it should be possible for even very high frequencies tobe measured. In order that the invention may be clearly understood and readily carried into effect it will now be described more fully with reference to the accompanying drawing, in which one construction of a high-frequency voltmeter accordin to the invention is illustrated.

In the embodiment shown, the high-frequency voltage to be measured is supplied, via input terminals l and a coupling condenser 2, to the control grid of an amplifier valve 3 whose anode circuit includes a resistor l of such low value that the amplification obtained to the desired high limit-frequency does not vary with the fre quency of the high-frequency signal.

The amplifier-output voltage is supplied to a modulator M which is constructed as a capacitative potentiometer one end of which is connected to the anode of the amplifier valve 3, whereas the other end is grounded. The potentiometer comprises a fixed condenser 5 anda condenser 6 which can be varied mechanically for example by movement of one of the condenser electrodes. The movement maybe brought about by means of a unilaterally 'fixed Spring Whose free end is set vibrating by means of an excitation coil 1 supplied from a source of auxiliary voltage such as an alternating current power line. As an alternative, for the movement of the condenser electrode use may be made, say, of a loudspeaker system which for example with the aid of a tube generator is driven with a frequency that'preferably diverges from the usual power line frequency and its harmonics.

The output voltage of the potentiometer is obtained from the condenser 6 and after detection by detector 8 including a diode rectifier and selective amplification by amplifier 9 is supplied to an indicator ll] of the usual type.

The output voltage established across condenser 6 takes the form of the applied high frequency voltage as amplitude modulated at the frequency of the auxiliary voltage. Appearing across the cathode resistor in the diode circuit of detector 8 is a measuring voltage whose frequency corresponds with the frequency of the auxiliary voltage and whose-amplitude is proportional to the amplitude of the high frequency voltage. The condenser shunted across the oathode resistor in detector 8 by-passes the high frequency components present in the detected signal.

In the embodiment shown, thevariable potentiometer acting as a modulator is built up from two condensers. Essentially, however, it may be constituted by inductances or ohmic resistances or else by impedances of differing kind. In the embodiment described, a simple and thus cheap unit may, however, be realized in practice, said unit acting in a manner free from interference and the fixed condenser 5 having at the same time the function of a blocking condenser between the anode circuit of the amplifier stage and the detector 9.

It may finally be mentioned that it is not necessary to cause the potentiometer ratio. of the potentiometer to vary gradually, for example in accordance with a sine function. As an alternative, the variation of the potentiometer ratio may be sudden, for example by periodically shortcircuiting one of the constituting impedances of the potentiometer or byperiodically interrupting the potentiometer by means of a vibratory or rotary contact of a type known per se.

What I claim is:

1. An aperiodic high frequency voltage measuring system comprising a capacitative potentiometer constituted by a fixed condenser connected in series with a variable condenser, means to vary periodically at a predetermined rate the value of said variable condenser, means to apply the high frequency voltage to. be measured. across the series-connected condensers, meanswfor detecting the resultant voltage. established. across said variable condenser to produce a measuring voltage whose amplitude isproportional to the amplitude of said high frequency voltage and whose frequency corresponds with said predetermined rate, and means to indicate the amplitude of said measuring voltage.

2. An aperiodic high frequency voltage measuring system comprising a capacitative potentiometer constituted by a fixed condenser connected in series with a variable condenser having an adjustable electrode, a source of low frequency auxiliary voltage, an electromagnet energized by said'source and arranged to actuate periodically said adjustable electrode at a rate-in accordance with the frequency of said source, means to apply the high frequency voltage to be measured across the series connected condensers, means for detectin the resultant voltage established across said variable condenser to. produce a measuring voltage whose amplitude is proportional to the amplitude of said high frequency voltage and whose frequency corresponds to the frequency of said auxiliary voltage, and means to indicate the amplitude of said measuring voltage.

3. An aperiodic high, frequency voltage measuring system comprising a capacitative potentiometer constituted by a fixed condenser connected in series with a variable condenser, means to vary periodically at a predetermined rate the Value of said variable condenser, an aperiodic amplifier for amplifying the high frequency voltage to be measured, means for applying the amplified high frequency voltage across the series-connected condensers, means for detecting the resultant voltage established across said variable condenser to produce a measuring voltage whose amplitude is proportional to the amplitude of said high frequency voltage and whose frequency corresponds with said predetermined rate, a selective amplifier for amplifying said measuring voltage, and means for indicating the amplitude of said amplified measuring voltage.

4. In an aperiodic high frequency voltage measuring system a modulator comprising a capacitative potentiometer constituted by a fixed condenser connected in series with a variable-condenser having an adjustable electrode, a source of low frequency auxiliary voltage, an electromagnet energized by said source and arranged to vibrate said adjustable electrode at a rate in accordance with the frequency of said sou-roe, and means to apply the, high frequency voltage to be measured across the series-connected condensers whereby the resultant-voltage established across said variable condenser has an amplitude proportional to said high frequency voltage and a frequency in accordance with said predetermined rate.

HENDRIK JAN LINDENHO-VIUS..

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES.

Donker, July 13, 1943, Device-for Measuring Direct Voltages, A. P. C. No. 270,105.

Havenhill et al., November 1944, Electrostatic Properties of Rubber and GRFSR 

